1. Technical Field
The present invention relates to a waveform compressing apparatus for compressing a waveform data, a waveform decompressing apparatus for decompressing a compressed data, and a method of producing a compressed data.
2. Background Art
In case that a waveform data is recorded to a waveform memory used in an electronic musical instrument or the like, there is known a technology of reducing a capacity of the waveform memory by compressing the waveform data. As systems of compressing the waveform data, there are known a scalar quantizing system and a vector quantizing system. According to the scalar quantizing system, “1” sample of an instantaneous value of the waveform data is made to correspond to “1 code” of the compressed data, and according to the vector quantizing system, a plurality of samples of instantaneous values of the waveform data are made to correspond to “1 code” of the compressed data.
In a waveform memory sound source of a background art, an adopted quantizing system is the scalar quantizing system, and the vector quantizing system is not adopted. This is because the waveform data of musical instrument sound changes over time in a characteristic of the waveform, and therefore, it is difficult to find out a characteristic common to a total of the waveform data (correlation among instantaneous values). Thus, even if the vector quantizing system is adopted, it is difficult to achieve an advantage of promoting a compression rate. For example, JP-A-2004-294491 discloses a waveform memory sound source which subjects a waveform data to linearly predicted compression in a unit of a frame by a waveform compressing apparatus to thereby provide a compressed waveform data of a scalar quantizing system, and which stores the compressed waveform data to the waveform memory.
Meanwhile, by compressing the waveform data used in the waveform memory sound source, an economic effect achieved by promoting the compression rate is very remarkable. That is, copies of ROM recording the waveform data are mass-produced for the waveform memory sound source apparatuses, and therefore, when data amount of respective waveform data can be reduced even by small amounts, a significant economic effect is achieved as a whole.
On the other hand, in compressing the waveform data for a sound source apparatus, there is no need for real time performance. That is, when the waveform data for a sound source is compressed, even if a compressing process is carried out by consuming a time period exceeding a time length of the original waveform data, any problem is not particularly brought about.
In view of such a situation, when the waveform data for the sound source is compressed, even if a long time period is consumed, it is preferable to search a compression mode capable of reducing a data amount significantly.
Further, when musical instrument sound recorded as a waveform data for a sound source apparatus is observed, there is a tendency that at an attack portion where the waveform is disturbed, a correlation among sample values is reduced, and that at a steady-state portion where the waveform is stabilized, the correlation among the sample values is increased. Further, the tendencies significantly differ by a kind of a musical instrument. Therefore, when musical instrument sound is compressed to record by a unit of a frame, it is preferable not to apply fixedly a certain quantizing system or compressing mode but to adopt an optimum quantizing system for each frame. Further, when the vector quantizing system is adopted, it is conceived to be further preferable to apply an optimum one of a code book or the like for each frame.